The structure of a piezo resistor-type three-axis acceleration sensor will be described below. FIG. 14 shows an exploded perspective view of a conventional acceleration sensor stated in Patent Document 1: Japanese Laid-open Patent 2004-184081. In an acceleration sensor 1, a sensor element 2 is fixed to a protective case 3 with adhesive. A protective case lid 4 is fixed to the protective case 3 with adhesive. Sensor terminals 6 of the sensor element are connected to the case terminals 7 of the protective case by wires 5. The output of the sensor element 2 is delivered outward through external terminals 8.
The following is a description of a sensor element used in a conventional piezo resistor-type three-axis acceleration sensor. Hereinafter, unless otherwise specified, the same reference characters are used for the same components and portions in order to simplify the description. FIGS. 15A and 15B show a schematic plan view of the sensor element 2 and layout of piezo resistors stated in Patent Document 2: Japanese Laid-open Patent 2003-279592 respectively. In FIG. 15, metal lead wires for connecting between the piezo resistors and sensor terminals thereof and the sensor terminals are omitted for convenience of understanding. The sensor element is composed of a weight 11 formed by a thick part of a silicon single crystal substrate, a support frame 10 so arranged as to surround the weight, two pairs of beam-shaped flexible arms 12, formed by a thin part of the silicon single crystal substrate, orthogonal to each other for connecting the weight 11 to the support frame 10, and the piezo resistors, each provided so as to correspond to the two orthogonal directions (X and Y) on the flexible arm and to the direction (Z) perpendicular to the flexible arm, consisting of X-axis piezo resistors 14, Y-axis piezo resistors 15 and Z-axis piezo resistors 16 in FIG. 15. A through-hole 13 is provided in the thin part of the silicon single crystal substrate so that the flexible arms 12 are in the shape of a beam, thereby the flexible arms 12 are easy to deform and suitable for high sensitivity. Both the X-axis and the Y-axis piezo resistors are the same as each other in output detection principle, connection method and layout, so that they are interchangeable. Hereinafter, unless otherwise specified, the flexible arms extending in the horizontal direction in the figures are taken as X-axis. The piezo resistors provided on the X-axis are referred to as X-axis piezo resistors 14. The Z-axis piezo resistors 16 are regarded as being provided on the same flexible arms as the X-axis piezo resistors 14. FIG. 15B is a partially enlarged view of FIG. 15A.
To increase the acceleration detecting sensitivity (output) of the sensor element, the flexible arms 12 need to be further lengthened, narrowed, and thinned, and the weight 11 needs to be further weighted so that the flexible arms are largely deformed by a slight external force. The terminals of the piezo resistors are aligned with connection ends 17 of the flexible arms being the maximum stress part to effectively take out the quantity of deformation of the flexible arms as the quantity of change in resistance of each axis piezo resistor. The connection ends 17 of the flexible arms are connections between the support frame and flexible arms or between the weight and flexible arms, borders between the flexible arms bent by an external force and the support frame or the weight that is not bent nor moved by the external force, and a position where a maximum stress point occurs on the flexible arm. In most cases, the X-axis piezo resistors 14 and the Z-axis piezo resistors 16 are arranged on the same flexible arm, so that they are symmetrically arranged with respect to the width centerline of the flexible arm, while the Y-axis piezo resistors 15 are arranged on the width centerline. Symmetrical arrangement of the piezo resistors with respect to the width centerline facilitates designing and producing the metal wirings for connecting between the piezo resistors.
An acceleration sensor mounted on mobile devices and the like is required to detect acceleration of several G's. It is important that the sensor element does not break down even if the acceleration sensor is subjected to acceleration of about 3000 G when the mobile device is dropped. It is necessary to resolve a contradiction that an easily-bendable flexible arm is needed to raise the acceleration detection sensitivity of the sensor element, but on the other hand, a less-deformable flexible arm is needed to raise a shock resistance by increasing its mechanical strength.
Patent Document 3: Japanese Laid-open Patent 4-274005 sets forth a configuration for restricting the movement in the Z-axis direction to increase a shock resistance. To realize this, regulating plates 18 and 19 are mounted above and below a sensor element with predetermined gaps of g1 and g2, as shown in a cross-sectional view of FIG. 16. When an acceleration sensor is subjected to shock, a weight 11 contacts regulating plates 18 and 19 to prevent a flexible arm 12 from breaking down before it is deformed to result in breakdown. The movement amount in the Z-axis direction is restricted within the values of g1 and g2. The movement amount in the X-axis and the Y-axis direction is restricted by corners of the weight contacting the regulating plate 19. It is difficult to set an optimum gap g2 because the amount of movement in the X-axis and the Y-axis direction until the corners of the weight 11 contacting the regulating plate 19 is changed, depending on the torsion of the flexible arm, but the regulating plate is effective as a structural element to raise a shock resistance.
Patent Document 4: Japanese Laid-open Patent 2002-296293 describes a structure for restricting the movement of a flexible arm by a part of a weight of a sensor element contacting other parts. FIG. 17 is a perspective view of the sensor element 2 disclosed in Patent Document 4. This structure is basically the same as that of the sensor element previously described in FIG. 15. A weight 11 is provided with petal-shaped auxiliary weights 22. When the sensor element is subjected to shock in the X-axis or the Y-axis direction, sides of the auxiliary weights 22 contact side walls of a support frame 10 to prevent flexible arms 12 from breaking down before they are deformed to result in breakdown. Addition of the auxiliary weights 22 increases the mass of the weight as a whole, so that this structure is effective to raise an acceleration detection sensitivity (output) of the sensor element, but the flexible arm liable to break down according to the mass increasing. In particular, it is difficult to stop an excessive movement in the Z-axis direction only by adding the auxiliary weights 22, which needs using a regulating plate as disclosed in Patent Document 3 as well.
The structures for improving the shock resistance disclosed in Patent Documents 3 and 4 are not intended to increase mechanical strength of the flexible arm. Patent Document 5: Japanese Laid-open Patent 64-18063 sets forth a structure in which a bend is provided at a connection between a flexible arm 12 and a support frame 10 or between the flexible arm 12 and a weight 11. FIG. 18A, FIG. 18B and FIG. 18C show a plan view, a cross-sectional perspective view taken on the line 18B-18B of FIG. 18A, and a plan view of another embodiment, respectively. FIGS. 18A and 18B show a structure in which a bend 24 is provided in the thickness direction of the flexible arm 12. FIG. 18C shows a structure in which a bend 25 is provided in the plane direction of the flexible arm 12. The provision of the bends 24 and 25 disperses stress applied to the bends uniformly to the whole bends and increases the mechanical strength of the flexible arm 12. In FIG. 18, strain gauges 23 are used to detect stress. Nothing has been mentioned about a relationship in position between the strain gauges 23 and the bends 24 and 25.
Patent Document 6: Japanese Laid-open Patent 8-29446 sets forth an acceleration sensor, which has bends (shape changing parts 26) provided in the plane direction of a flexible arm as in the case with Patent Document 5, and in which a positional relationship with a piezo resistor is clarified. FIG. 19A is a plan view of the acceleration sensor, and FIG. 19B is a cross section taken on the line 19B-19B of FIG. 19A. Connection of the flexible arm 12 supporting a weight 11 to a support frame 10 through the shape changing part 26 causes a maximum stress part to occur on a border between the flexible arm 12 and the shape changing part 26. On the border is located a piezo resistor 27. The shape changing parts intervening between the flexible arm and the support frame reduce stress on the border between the support frame and the shape changing parts, improving shock resistance. Judging from the existence of the maximum stress part on the border between the flexible arm 12 and the shape changing parts 26, the shape changing parts are treated as parts that are not bent and do not move even if they are subjected to an external force. In Patent Document 2, a bending part is the flexible arms, and an unbending part is the support frame, while, in Patent Document 6, a bending part is the flexible arm and an unbending part is the shape changing parts and the support frame, so that it is apparent that the positions of the maximum stress part and layout of the piezo resistor are the same for the Documents.
The above is a description of the structures of the acceleration sensors of which shock resistances are improved, disclosed in the prior technical documents. However, these structures have some drawbacks and advantages as measures for improving shock resistances. The structure, in which the regulating plates are provided as described in Patent Document 3, requires adding the regulating plates as components and accurately assembling the plates. Addition of the regulating plates makes it difficult to thin the acceleration sensor. It is easily understandable from Patent Document 4 that it is difficult to accurately produce a slight space between the auxiliary weight and the support frame in using a silicon substrate that is as thick as the weight. In the structure of Patent Document 6, the piezo resistor is provided on the maximum stress part to maximize the output. However, the shape changing parts are taken as an unbending part, which should substantially shorten the length of the flexible arm if the outer dimension of the sensor element and the width of the support frame are unchanged, result in decreasing the absolute value of output. Enhancement of the absolute value of the output needs increasing the outer dimension of the sensor element, which makes it difficult to downsize.
In a three-axis acceleration sensor, it is necessary to balance output of each axis. A great difference between outputs of the axes needs preparing an amplifier different in amplification factor for each axis, which increases cost. Moreover this widens circuit area and may impede downsizing. In particular, difference between outputs of the X-axis (Y-axis) and the Z-axis remains a major problem. FIG. 20 is a graph showing a relationship between a weight thickness and outputs of X-axis and Y-axis. The Z-axis varies according to a linear function with respect to the thickness of a weight. The X-axis varies according to a quadratic function with respect to the thickness of the weight. The outputs of the X-axis and the Y-axis approximately equal to each other at a thickness of about 800 μm. The prevailing silicon single crystal substrate used in semiconductor manufacturing is 625 μm or less in thickness, but an about 800-μm thick substrate is disadvantageous in price and delivery. It is inevitable that the sensor element will be thinned. Thinning further enlarges the difference between outputs of the X-axis and the Y-axis.